The invention relates to a method and a device for ascertaining the fill level in vessels, wherein mechanical vibrations are produced without contact in a vessel wall, the produced mechanical vibrations are recorded without contact and the recorded vibrations are analyzed.
Such a method is known from GB 2 298 279 A, based on the finding that, in gas vessels of a certain type, it is not the resonance frequency, but only the frequency amplitude, that changes depending on the contained residual amount. To establish the residual amount, sound waves of this resonance frequency are therefore directed onto the vessel by means of a loudspeaker and the intensity of the reflected sound waves of this frequency, is ascertained and compared with a threshold value.
It is known from DE 40 04 965 A1 to test vessels for the tightness of attached below-atmospheric pressure closures, by producing mechanical vibrations without contact in the below-atmospheric pressure closure by a short time magnetic field and evaluating the produced vibration with regard to frequency, duration and/or attenuation.
It is known from U.S. Pat. No. 3 802 252, U.S. Pat. No. 4 811 595, U.S. Pat. No. 5 353 631 and GB-A-2 293 450 to ascertain the internal pressure of a vessel by mechanically or magnetically impacting a vessel wall and measuring the resonance frequency of the vessel, from which the internal pressure is then derived.
A method for establishing a minimum and maximum fill level is known from DE-A-40 08 135 in which the resonance frequency is ascertained in each case at a certain point on the external wall of the vessel. Piezo- crystal are used to produce the vibrations and to scan the vibrations.
A method is known from DE-A-41 00 338 in which the degree of the filling of the vessel with the free-flowing product is ascertained by measuring the frequency of the mechanical sound vibrations of the vessel housing by a sensor fitted directly onto the vessel wall.
A method is known from DE-A-197 11 093 (=EP 0 831 308 A) in which the contents of a liquefied gas bottle are determined by directing a sound signal onto the gas bottle by means of a sound transmitter placed against the outside of the gas bottle, recording the sound signal given off by the gas bottle, and comparing both signals. The transmitted signal passes through a substantial frequency range and the resonance frequency is established by measuring the amplitude of the received signal.
A method is known from WO 94/24526 in which even ultrasonic waves are produced in a liquid sample in a vessel and the dependence on frequency of the amplitude and/or the phase of the resonances are measured.
In the earlier application DE-A-196 46 685 (=WO 98/21557) which is still not published, a method for determining the fill level of closed vessels is described, primary mechanical vibrations being excited in a vessel wall and the secondary vibrations which are excited by the primary mechanical vibrations of the vessel wall inside the vessel and occur inside the space between the closure and the liquid then being analyzed. The fill level can be ascertained from the frequency of these secondary vibrations.
In the earlier application DE-A-197 36 869 (=WO 99/10722) which is still not published, a method for testing the residual air volume of vessels, which are sealed by a closure is described. The liquid for expulsion of the residual air volume is foamed in the vessels before sealing. The residual air volume is ascertained by exciting mechanical vibrations in the closure which are analyzed directly after the closure is attached, before a major change in the internal pressure takes place in the vessel. The frequency, the decay time, the vibration amplitude and/or the time integral of the vibration amplitude are included in the vibration analysis.
The object of the invention is to determine the fill level of vessels, in particular cans, in as easy and rapid a manner as possible.
This object is achieved according to the invention in that the vibration is produced in a vessel wall which is contacted by the contents to an extent which varies depending on the fill level and in that recorded vibrations are then analyzed to discover to what extent the vessel wall is contacted on the inside by the contents, this including the evaluation of the decay time, the frequency, the intensity and/or the time integral of the intensity or the ascertaining of the site of the maximum intensity of the mechanical vibration.
The mechanical vibrations in the vessel wall are produced in known manner by a short magnetic pulse emanating from a magnetic coil. The magnetic pulse briefly deflects the vessel wall and the vessel wall vibrates back after the magnetic pulse has ended. The mechanical vibration of the vessel wall produces acoustic vibrations, which, for their part, can be received by a microphone, magnetic recorder or the like. This measuring technique has long been known in connection with the determination of the internal pressure of vessels. These mechanical vibrations decay with a certain time constant. The time constant is relatively small, i.e. the attenuation is great when the vessel is full. The lesser the contents, the greater the time constant of the vibration. The fill level has a particularly significant effect on the vibration attenuation.
The increased attenuation also has an effect on the time integral of the vibration amplitude. This time integral is proportional to the area under the curve representing the vibration in a diagram showing the vibration amplitude over time. The smaller this area, the higher the fill level.
In addition, shifts in the vibration frequency also result which can likewise be evaluated. It is generally the case that the frequency becomes higher as the fill level rises.
Because the mechanical vibrations are produced without contact e.g. by a magnetic pulse, in the vessel wall and the vibrations are also recorded without contact, the method according to the invention is suitable in particular for ascertaining the fill level of vessels which are conveyed along a conveyor belt or other transport device.
In a preferred version of the invention, the vibrations of the vessel wall are recorded by two microphones arranged at a distance one above the other and the site of the sound source is ascertained by a comparison of the vibration phases. This exploits the fact that the vibrations in the vessel wall are markedly attenuated below the surface of the liquid, so that it is principally the part of the vessel wall lying above the surface of the liquid that vibrates, so that the centre of the vibrations produced in the vessel shifts upwards as the fill level rises. Information about the fill height can therefore also be obtained by locating the sound source.
The distance between magnetic coil and vessel as well as between vessel and microphone does not play a major role in the evaluation of the vibration with regard to attenuation and frequency. These two distances must however be taken into account when the vibrations are evaluated with regard to the time integral of the amplitude, as here the absolute intensity of the vibration affects the measurement result. Therefore, the distance between magnetic coil and vessel wall and between vessel wall and microphone are preferably taken into account in the evaluation of the vibration with regard to the time integral of the intensity or amplitude. The distance can be measured in known manner by a laser beam, inductively or by ultrasound.
Due to wave motions and sloshing effects of the liquid in the vessel, measurement inaccuracies result. To improve the measurement accuracy, several measuring devices are therefore preferably linked, each measuring device consisting of a magnetic coil and one or two microphones. As the costs of individual measuring devices are relatively low, a clear improvement in the accuracy can be achieved by a larger number, e.g., 10 or more, of measuring devices and the formation of the average value from the results of all measuring devices. Not only the inaccuracy caused by the sloshing of the liquid, but also the inherent measuring inaccuracies of the system, is reduced by a larger number of measuring devices. The measuring devices can thus be arranged on both sides of the vessel or the can.